Paclobutrazol inhibition of sterol biosynthesis in a cell suspension culture and evidence of an essential role for 24-ethylsterol in plant cell division.

Growth of a celery (Apium gravidens) cell suspension culture was inhibited by the synthetic plant growth regulator paclobutrazol. Paclobutrazol caused a reduction in the incorporation of [2-14C]acetate into the 4-demethyl sterols (campesterol, sitosterol, stigmasterol) but radioactivity accumulated in the 4 alpha-methylsterols. The accumulating 4 alpha-methylsterols were identified as obtusifoliol and cycloeucalenol indicating that paclobutrazol was inhibiting sterol biosynthesis by blocking 14 alpha-demethylation. The inhibition of celery cell growth by paclobutrazol could be partially overcome by addition of cholesterol to the culture medium. However, addition of stigmasterol restored growth to the control value suggesting an essential role for a 24-ethylsterol to support plant cell division.